1. Field of the Invention
The present invention relates to a novel lubricating oil for refrigerators with a compressor. More particularly, it relates to a lubricating oil for refrigerators with a compressor (compression-type refrigerators), comprising as the main component a polyoxyalkyleneglycol derivative having a high lubricating property as well as a favorable compatibility with hydrogen-containing fluorocarbon compounds such as 1,1,1,2-tetrafluoroethane (hereinafter referred to as R-134a), which can substitute for Freon compounds such as dichlorodifluoromethane (hereinafter referred to as R-12) used as refrigerant and involved in environmental pollution problems.
2. Description of the Related Arts
Generally, a compression-type refrigerator is constituted of a compressor, a condenser, expansion bulbs, and an evaporator, having a mechanism where the mixture of the refrigerant and the lubricating oil is circulating in a closed system. In such a refrigerator, though it depends on the kind of apparatus, usually the temperature in the compressor rises to 40° C. or more, while in the cooler, the temperature comes to be so low as −40° C. Accordingly, the refrigerant and the lubricating oil must circulate in the said system without phase separation in the said temperature range of −40° C. to +40° C. in most cases.
If phase separation is caused while the running of the refrigerator, a seriously detrimental effect occurs affecting the life and efficiency of the apparatus. For example, if phase separation of the refrigerant and the lubricating oil is caused in the compressor, moving parts become underlubricated resulting in seizure or other troubles, which shorten the life of the apparatus considerably. If phase separation is caused in the evaporator, a lubricating oil having a high viscosity results, which brings lowering of efficiency in heat exchange.
Since a lubricating oil for refrigerators is used for the purpose of lubricating the moving parts of the refrigerator, its lubricating ability is regarded important as a matter of course. Particularly in the compressor, the temperature becomes so high that a viscosity, sufficient to retain the oil film necessary for lubricating, is important. Necessary viscosity varies with the kinds or running conditions of the compressor, but usually the preferable viscosity of the lubricating oil before blended with refrigerant is 2 to 250 cSt at 100° C. If the viscosity is lower than that range, the oil film becomes thinner thereby causing underlubricating and sealing properties become poor, while if the viscosity is higher than the above, the efficiency in heat exchange is reduced. Since lubricating oils for refrigerator are used in circulation in a wide range of high to low temperatures, its viscosity index is preferably high, and usually a viscosity index of 40 or more is required. Further, in addition to the above properties, a low hygroscopicity to prevent the blocking of the bulb caused by icing at the expansion bulb and other properties are required.
Heretofore, as the refrigerant for compression-type refrigerators, R-12 has often been used, and as the lubricating oil, various mineral oils and synthetic oils satisfying the required properties mentioned before have been used. R-12, however, has recently been restricted more and more severely all over the world, for it is liable to cause environmental pollution including destruction of the ozonosphere.
In these circumstances, hydrogen-containing fluorocarbon compounds including R-134a have attracted attention, as new refrigerants. Among the hydrogen-containing fluorocarbon compounds, R-134a especially has only a small possibility of destroying the ozonosphere, and can substitute for R-12, with little change in structure of the conventional refrigerators, and accordingly is favorable as a refrigerant for compression-type refrigerators.
When the hydrogen-containing fluorocarbon compounds including said R-134a are employed as the refrigerant for compression-type refrigerators in place of R-12, the desirable lubricating oils come to be those having high compatibility with the hydrogen-containing fluorocarbon compounds including R-134a, and also having high lubricating properties enough to satisfy the required properties mentioned above.
However, since the conventional lubricating oils which have been used with R-12 do not have a favorable compatibility with the hydrogen-containing fluorocarbon compounds including R-134a, a new lubricating oil suitable to these compounds consequently is required. In that case, particularly in the air-conditioner for automobiles, it is required that the construction of the mechanism be hardly changed on the occasion of substituting for R-12. It is not desirable to change widely the construction of the present mechanism because of lubricating oil. Accordingly, a lubricating oil having a very favorable compatibility with the hydrogen-containing fluorocarbon compounds, including R-134a, is required.
As lubricating oils having compatibility with R-134a, Ulcon LB-165 or Ulcon LB-525 (Trade Mark, both produced by Union Carbide Co., Ltd.) composed of polyalkyleneglycol have been known, and it was reported that these lubricating oils are dissolved or compatible with R-134a in all proportions at a low temperature of −50° C. or lower (“Research Disclosure”, No. 17463 (October, 1978)). Moreover, a high viscosity refrigerator oil composition of which the base oil is poly-oxypropyleneglycolmonobutylether is known (Japanese Patent Publication No. 42119/1982). These lubricating oils are polyalkyleneglycol derivatives having polypropylene glycol with a hydroxyl group at one of the terminals and a n-butyl ether bond (a n-butoxy group) at the other terminal. They have a comparatively favorable compatibility with R-134a at low temperatures, but they do not have sufficient compatibility with R-134a at high temperatures, and for example, Ulcon LB-525 mentioned above is known to cause phase separation with R-134a at room temperature (Specification of U.S. Pat. No. 4,755,316).
On the other hand, a polyglycol having at least two hydroxyl groups in a molecule is proposed to be a favorable substance compatible with R-134a (Specification of U.S. Pat. No. 4,755,316).
In the said polyglycol, however, the compatibility is not necessary sufficient.
Meanwhile, it is known that polyglycol generally shows a temperature dependency that when the mixture with a fluorocarbon or chlorofluorocarbon compound is heated from a low temperature to a high temperature, then the mixture phase-separated is once dissolved and phase-separated again.
On the other hand, R-134a and compounds which can dissolve it were proposed for use in absorption-type refrigerators (Japanese Patent Kokai No. 79175/1989). Said absorption-type refrigerators, however, are quite different in mechanism from the before-mentioned compression-type refrigerators, and tetraethylene-glycoldimethylether described in the Examples of the above-described Kokai is not proper as a lubricating oil for compression-type refrigerators because of its particularly low viscosity.
As described above, lubricating oil for compression-type refrigerators having sufficiently favorable compatibility with R-134a as well as high lubricating properties have not been found yet, and their development has been eagerly desired.
Replying to these requirements, the present inventors have earnestly studied to develop a lubricating oil suitable to compression-type refrigerators employing, as the refrigerant, hydrogen-containing fluorocarbon compounds including R-134a which are able to substitute for R-12 now involved in environmental pollution problems and other fluorocarbon compounds which are difficult to decompose.